Question

我可以看到select_related减少查询次数。

但是，当我多次运行相同的查询时，我希望看到第一个查询被重用，但它不在以下设置中。

class Foo(models.Model):

    pass


class Bar(models.Model):

    foo = models.ForeignKey(Foo, related_name='bars')
    blog = models.ForeignKey('blg.blog')



foo = Foo.objects.create()
bar1 = Bar.objects.create(foo=foo)
bar2 = Bar.objects.create(foo=foo)

foo.bars.select_related('blog')
foo.bars.select_related('blog') # <= here, it runs the query again

如何告诉django在第二次运行时重用第一个数据库查询？

Answer 1

select_related并不意味着任何缓存，只是执行JOIN，因此您不需要新的数据库查询来获取返回对象的相关对象。

for bar in foo.bars.all():
    bar.blog  # A query to fetch the blog object is made for each bar object in the loop.

for bar in foo.bars.select_related('blog'):
    bar.blog  # No query is made, the blog object has already been fetched.

如果您不想进行多个数据库查询，只需将查询集存储到变量中并重用相同的查询集，例如：

bars = foo.bars.select_related('blog')

for bar in bars:  # The first loop executes the DB query and caches its result
    # Do something

for bar in bars:  # The second loop uses the cached results
    # Do something else

Django，在反向相关管理器上重用select_related / prefetch_related？

1 个答案: